DESCRIPTION: (Adapted from Applicant's Application) Early intervention is imperative for successful treatment of progressive diseases. Hemopoietic stem cell (HSC) transfers in utero provide a window for therapeutic intervention when immunological barriers are not yet formed. Unfortunately, donor stem cells with no selective advantage in the host show delayed amplification, incomplete replacement, and an overall low frequency of long term repopulation in both human and animal systems. These severe limitations to in utero transplantation are independent of donor cell numbers, donor cell source, and donor cell seeding. The current proposal will test the hypothesis that the HSC seed but are unable to compete with a rapidly expanding host hemopoietic cell population in utero. The investigators will assess causes for therapeutic failures by elucidating the immediate and long term fate of individual transplanted donor cells in a new model system that combines useful aspects of two well characterized mutant mice. Enriched HSC will be transplanted in utero into genetically myeloablated W41/W41 fetuses deficient for the lysosomal enzyme beta-glucuronidase (beta-gus). The W41/W41 fetuses offer the same barriers to successful long term repopulation as the other models but their mild anemia confers a small selective advantage on the donor cells allowing them to seed. The beta-gus null status of the host enables immediate tracking of the donor beta-gus positive cells by histochemistry and flow cytometry. In addition, the beta-gus deficient mutant mouse, as a model for the rapidly progressive human lysosomal storage diseases that are candidates for in utero therapy, provides the opportunity to correlate transplantation therapy with disease status. The aims are to: identify, by histochemical staining for beta-gus, the sites immediately seeded by enriched +/+ HSC in utero; characterize and quantify, by flow cytometry, the donor hemopoietic progeny in the peripheral blood and in hemopoietic sites during successive days after in utero injection; determine the extent of the lysosomal storage and the effects of corrective therapy in utero; and identify treatment modalities that improve therapy for the progressive disease in the MPS VII mice.